Process for heat-treating electromagnetic steel sheets having a high magnetic induction

ABSTRACT

A method for producing a single-oriented silicon steel sheet having a very high magnetic induction by subjecting a steel sheet containing C and acid-soluble Al to the following process steps; rolling the steel sheet to an intermediate gauge, subjecting the rolled steel sheet to an annealing in a temperature range of 750* to 1,200* C. for 30 seconds to 30 minutes followed by quenching, thereby to cause N as AlN to precipitate in the steel sheet in an amount of at least 0.0005 percent and then coldrolling the quenched steel sheet.

O Y Unlted States Patent 1151 3,636,579

Sakakura et al. [4 1 Jan. 25, 1972 54] PROCESS FOR HEAT-TREATING 3,147,]58 9/1964 Fledler ..14s/111 ELECTROMAGNETIC STEEL SHEETS f HAVING A HIGH MAGNETIC 3:l8 4:346 5 1965 INDUCTION 3,214,303 10/1965 Fiedler 3,266,955 8/1966 Taguchi et al.. [72] inventors. Akiri Sakakurn, Satoru Taguch, Toshlya Wada; Kiyoehi Ueno; Takooki Yamalnoto; l 1/1966 Taguch' at 21:12:1 Urushiyama, a" of Knakyushu' Primary Examiner-L. Dewayne Rutledge Assistant Examiner-G. K. White [73] Assignee: Nippon Steel Corporation, Tokyo, Japan Attorney-Wenderoth, Lind & Ponack [22] Filed: Apr. 21, 1969 [57] ABSTRACT [2]] App! A method for producing a single-oriented silicon steel sheet having a very high magnetic induction by subjecting a steel [30] Foreign Application Priority Data sheet containing C and acid-soluble Al to the following process steps; rolling the steel sheet to an intermediate gauge, Apr. 24, Japan j g the toned Steel Sheet to an li g i a p ture range of 750 to l,200 C. for 30 seconds to 30 minutes [52] U.S. Cl .148/111, 148/112 followed by quenching, thereby to cause N as MN to [51] precipitate in the steel sheet in an amount of at least 0.0005 [58] held of Search ..l48/l l0, 1 l l, 112, 1 13 percent and then cold ro|ling the quenched Stee| Sheet [56] References Cited 1 Claims, 4 Drawing Figures UNITED STATES PATENTS 3,299 12 196; 1=ied ..14s/111 FIG. 1

PATENTED JAN25 I972 SHEET 1 UF 4 Mognetizing force H (De) (6) g uouanpu ouaubnw ATTORNEYS Pmmmmzsm $636579 sum u 0F 4 Time required for cooling down to 400 C (second) INVENTORS A kira Sakakura Ki yoshi Ueno oshi ya Wada aroru Taguchl Nobuv yrushiyama Takaak: amamoro BY? flwww (PM ATTORNEYJ PROCESS FOR HEAT-TREATING ELECTROMAGNETIC STEEL SHEETS HAVING A HIGH MAGNETIC INDUCTION This invention relates to a method for producing singleoriented electromagnetic steel sheets having an easy magnetization axis l in the rolling direction of the steel sheet.

Single-oriented steel sheets which are soft magnetic materials are used mostly as iron cores for transformers and other electric devices. Therein magnetic characteristics, excitation characteristics and core loss values must be favorable.

The present inventors use the magnetic induction B (gausses) generated in the iron core at the intensity H (oersted) of the magnetic field as a value for expressing the excitation characteristics, and the core loss W 15/50 (W/kg.) at 50 cycles and an alternating current magnetic induction of 15,000 gausses as a value for expressing the core, that is, an energy loss lost from the iron core when a prescribed altemating current magnetic induction is given to the iron core.

Recently, it has become an important problem to make the size of electric devices such as transformers small and to this end a reduction in the weight of the core is required. In general, in order to be able to reduce the weight of the core to be used for electric devices steel sheet, wherein the magnetic induction is high, must be used. Thus, a steel sheet having a favorable magnetization characteristics, that is, a steel sheet having high B characteristics, is needed, and a greater importance is attached to a steel sheet having a high saturated magnetic induction 13,. Further, it is noted that when using that portion of a steel sheet, wherein the magnetic induction is high, the core loss value generally increases, and as compared with a material having low B characteristics a steel sheet having B characteristics has a much lower core loss in the region of high magnetic induction, and moreover the increase in the core loss is at a lower rate as the magnetic induction rises.

Taking all these points together, it can be seen that an improvement in magnetic flux density which must be provided if the capacity of the electric devices is to be increased, can in the first instance be realized by the use of electromagnetic steel sheets having a very high magnetic induction.

The present invention has for its object the supply of products which can meet the requirements described above. That is, according to the method of the present invention it is possible to produce electromagnetic steel sheets which are markedly superior to any conventional single-oriented silicon steel sheets with respect to the magnetic induction 13 in the rolling direction, that is, steel sheets which have a magnetic induction as high as at least 18,500 gausses, and even up to 20,100 gausses. This indicates clearly that the magnetic induction of steel sheets of the present invention is far superior even to the highest value ever reported, that is B =l8,690 gausses disclosed in the U.S. Pat. No. 2,867,557 and B =l9,l00 to 18,000 gausses in US. Pat. No. 3,287,183, both granted to the present inventors. I

Further, the present inventors have discovered the following new fact in regard to the production of single-oriented electromagnetic steel sheets which have a low Si content or no Si content in order to improve B,, that is, B, is a physical property which is a determined by the chemical composition of an alloy steel and can not be influenced by treating conditions. Particularly, the Si content has a great influence on B,. For instance, when Si is 0 percent, B, is about 21,600 gausses, and when Si is 1 percent, B, is about 21,300 gausses, when Si is 2 percent, B, is about 20,800 gausses and when Si is 3 percent, B, is about 20,300 gausses. That is to say, the lower the Si content, the higher B,. However, heretofore, in conventional silicon steel sheets the reduction in resistance and the deterioration of core loss values were due to the low content of Si. Even apart therefrom, it was not possible to obtain on an industrial scale steel sheets having crystals with a so-called {1l0} l00 -orientation. Consequently single-oriented steel sheets could not be obtained and their high B, value could not be utilized. The present invention has succeeded in providing a method for manufacturing ideal single-oriented electromagnetic steel sheets with high magnetic induction, that is, a high B value, and moreover having a high B, value on account of the Si content being low, by producing secondary recrystallization grains having a very well selected {1 l0} l00 -orientation, extending over a wide range of silicon content from 0 to 4 percent.

In the U.S. Pat. No. 2,1 13,537 there is disclosed a method for producing an oriented silicon steel sheet by subjecting a silicon steel material containing 3.5 percent Si, 0.] percent Mn and 0.1 percent Alto the following production steps, that is, the silicon steel material is hot-rolled and then annealed at 1,000 C. for 1 hour and then quenched, then subjected to a series of cold-rollings, thereby to reduce the steel sheet to the final gauge and finally is warm-rolled at 450 C.

Further, in the US Pat. No. 3,151,005 is described a method for improving the core loss by annealing a hot-rolled silicon steel sheet containing at least 0.01 percent C at l,450 to 1,750" F. to effect the solid solution of C thereby and then quenching the steel sheet from this temperature ti) lower than 1 ,000" F.

The treating method according to the present invention relates to a method for producing single-oriented silicon steel sheet having a high magnetic induction, in which a normal steel material or silicon steel material which contains C and Al as indispensable elements is produced according to known steelmaking methods, melting methods and casting methods which are used as normal industrial techniques, the thus obtained steel material is hot-rolled and then subjected to an annealing process and cold-rolling process one time or more than one time respectively to reach the final gauge, the thus obtained product is decarburized and then subjected to a final annealing to generate secondary recrystallization grains having a {1l0} l00 -orientation in the steel material. The present invention modifies this conventional method in that the final cold-rolling is carried out at a reduction rate of 65 to percent depending upon the Si content and an intermediate annealing, preferably an annealing immediately before the final cold-rolling, is carried out in such a temperature range, that is, in the range of 750 to l,200 C., that 'y-transformation can occur at least in a part of the steel material, that is, in the range of 750 to 1,200 C., and thereupon the quenching is carried out from this temperature range, in which the transformation of 'y to a has been completed, to a temperature below the said range, that is, to a temperature below the range from 750 to 950 C. according to the Si content so as to cause AlN of preferred size to precipitate in the steel sheet, so that after the final annealing the magnetic induction B in the said rolling direction may well be at a high level, that is, at least 18,500 gausses and at a maximum 20,100 gausses.

The present inventors have already reported in US. Pat. No. 3,287,183 that a single-oriented silicon steel sheet having a high magnetic induction can be obtained from a silicon steel ingot having 2.5 to 4 percent Si and which contains C and Al as a starting material. By adding a new idea to this process, however, they have now succeeded in greatly improving the characteristics of the product and at the same time in producing a single-oriented silicon steel sheet having a high magnetic induction from a steel sheet having no silicon or a low silicon content, which has heretofore never been reported.

An object of the present invention is to provide a method for producing a single-oriented silicon steel sheet having a high magnetic induction.

Another object of the present invention is to provide a method for producing a single-oriented silicon steel sheet having a high magnetic induction from a steel sheet having no silicon or a low silicon content A further object of the present invention is to provide a chemical composition most suitable for producing a singleoriented silicon steel sheet having a high magnetic induction from a steel sheet having no silicon content or a low silicon content. 7,

The contents of the present invention will be made clear by reference to the following explanation and attached drawings.

FIG. 1 is a graph showing a comparison of the excitation characteristics of products of the present invention with those of known products on the market.

FIG. 2 is a graph showing a comparison of the core loss values of products of the present invention with those of known products on the market.

FIG. 3 is a graph showing the relationship between the C- acid-soluble Al values of products of the present invention and the excitation characteristics.

FIG. 4 is a graph showing the relationship between the cooling rate after the annealing according to the present invention and the excitation characteristics.

' The normal steel or silicon steel material which is a starting material in the present invention is an ingot made by solidifying by any casting method a molten steel made by a steelmaking method which is already known such, as, for example, by an open-hearth furnace, electric furnace or converter or melted by a known melting method such as, for example, by a high-frequency electric furnace or vacuum-melting furnace. A slablike ingot obtained by a continuous casting method, which recently has come into wide use, can also be used as a material in the present invention. The atmosphere in the case casting is usually air but may be vacuum or of an inert gas as well.

As described above, the material of the present invention can be made by any steelmaking, melting and casting methods. But the composition of the material must satisfy the following conditions, irrespective of the methods for producing the same, that is, steelmaking, melting or casting methods.

C less than 0.085 percent Si percent to 4.0 percent Al 0.010 to 0.065 percent Al, means an acid-soluble A1, which, however, will be hereihafter referred to simply as Al. The rest is Fe and unavoidable impurities. It is necessary that C in the above-mentioned material should be present in an amount sufficient to produce a -y-transformation at least in apart of the steel in response to the Si content. According to our experiences, C in a steel ingot must be at least 0.025 percent where Si is 3 percent but may be about 0.005 percent in case Si is 0 percent. As regards the added elements, the inventors have the following view. Generally, in the production of a single-oriented electromagnetic steel sheet, because a secondary recrystallization in the {l 10} l00 direction occurs inthe final annealing, a selected direction will be obtained. However, in a case, such precipitate produced by the slight added amount of a material such as, for example, nitride, sulfide or oxide will play an important role. Such role has been considered to merely finely disperse and precipitate the precipitate so that the normal grain growth will be inhibited and the secondary recrystallization will be accelerated. However, the present inventors have discovered that, besides the above-mentioned role of the precipitate, a part of the precipitate which has been precipitated with a specific directional relation relative to the matrix further has the capacity of selectively growing crystal grains only in a specific direction, which strictly regulates the direction of the secondary recrystallized grains so that, as a result, a product having excellent B characteristics can be obtained. AlN which produces a satisfactory effect by the addition of Al at the time of the final annealing in the present invention is of the latter type, thus the fonnation of AlN of such a type forms the basis of the present invention. In general, precipitates formed by the addition of other elements have not such an ability, but seem to have only the effect of restricting the growth of matrix crystal grains. However, the restricting effect of the general precipitates is also an important factor for effecting the secondary recrystallization. Therefore, the presence of precipitate-forming elements is permissible so far it does not hinder the formation of the said AlN. For instance, as is well known in the production of a singleoriented silicon steel sheet, S, Se and the like may be present in an amount of less than a maximum of 0.1 percent respectively. Further, according to the knowledge of the present inventors Te may be included in an amount less than 0.20 percent. However, in the present invention care must be taken unless such elements form precipitates like carbide. That is to say, it is necessary that Zr and Ti which are elements stronger than Al in affinity for N, would be added in proportion to the percent of N in the steel only after taking into consideration that AlN can be precipitated in a specified amount after the annealing as described later. B, Ta, Nb, V, Cr, Mn, W and Mo are weaker than Al in their affinity for N and therefore may be added in proper amounts already known in the production of singleoriented silicon steel sheets. The allowable maximum values of the amounts of these precipitate-forming elements which can be added are 1 percent for V, Mn and M0, 0.5 percent for W and 0.1 percent for B, Zr, Ti, Nb, Ta and Cr. However, these are all only examples. It is not a deviation from the idea of the present invention to add precipitate-forming elements in order to produce precipitates for the acceleration of the secondary recrystallization within the range in which the formation of AlN which is the basis of the present invention is not obstructed.

The reasons for specifying the composition of the ingot in the present invention are explained in the following. A silicon steel ingot, which contained about 1.8 percent Si and in which the contents of C and Al varied, was hot-rolled to a steelsheet about 2.0 mm. thick. The sheet was at first annealed in N at 1,050" C. for 2 minutes and then sprayed with atomized water drops. After the steel sheet was cooled down to a room temperature in about 50 seconds, it was cold-rolled to a steel sheet 0.35 mm. thick. The thus cold-rolled steel sheet was decarburized at 800 C. and was finally subjected to a box-annealin g at 1,050 C. for 15 hours. FIG. 3 shows the relationship between the magnetic, induction B of the thus obtained product and the amounts of C and Al in the ingot. As is evident from this figure, product according to the present invention in which the magnetic induction B in the rolling direction is higher than 18,500 gausses can be obtained when the contents of C and Al are in the ranges:

C 0.015 to 0.085 percent Al 0.010 to 0.065 percent Further, it became evident from the results of the same experiments in which the Si content was, however, varied so or to be 0 percent, 1 percent and 3 percent that the amount of Al necessary for obtaining a product having a magnetic induction B of more than 18,500 gausses was the same in all cases of different Si contents, but the amount of C should be less than 0.085 percent respectively or in a range of 0.025 percent to 0.085 percent. This corresponds to the amount of C required for effecting the 'y-transformation at least in a part of the steel sheet at the time of annealing before the final cold-rolling, as will be hereinafter described.

Si is kept less than 4 percent. The present invention has it as an object to improve the B characteristic and B, characteristic. Therefore, there is no lower limit.

Next, a method for producing a desirable AlN, which forms the basis of the present invention, will be explained. The present inventors have already explained in U.S. Pat. No. 3,287,183 that it was necessary to precipitate AlN having a precipitate size such that it was able to produce secondary recrystallization grains in the steel before the final coldrolling, and as one of the operating conditions for achieving the said object the annealing immediately prior to the final cold-rolling should be carried out within a temperature range of 950 to 1,200 C. for 30 seconds to 30 minutes when the Si content was 2.5 to 3.5 percent. In this case the C content in the steel before the said annealing was said to be in a range of 0.020 to 0.080 percent. In the present invention, the principal object of which is to obtain a single-oriented silicon steel sheet having a low Si content there are some variations in the said operating conditions according to the Si content. For instance, the annealing should be carried out in the temperature range of 850 to 1,200 C. when the Si content is l to 2.5 percent and in the temperature range of 750 to l,200 C. when the Si content is less than 1 percent. However, in the present invention substantially the same annealing conditions as those above mentioned are also required, although there can be some variations in the temperature range depending on the Si content as above mentioned. The annealing time can be in the range of 30 seconds to 30 minutes for any Si content. In any case the annealing time and the temperature range are such so as to be able to effect the 'y-transformation at least in a part of the steel sheet according to the Si content. After the annealing immediately prior to the final cold-rolling has been carried out under the operating conditions as above mentioned, the steel sheet is cooled down to a desired temperature and thereupon quenched by using any adequate artificial means.

FIG. 4 shows relationship between the magnetic induction B of a product and the cooling rate when the product is produced from a silicon steel ingot containing 2.2 percent Si, 0.045 percent C and 0.025 percent Al by the following steps, that is, blooming and hot-rolling the said steel ingot to a hotrolled steel sheet 2.3 mm. thick, then annealing the hot-rolled steel sheet in nitrogen for 2 minutes at each temperature as shown in FIG. 4 and thereafter gradually cooling the annealed steel sheet down to 850 C. at a certain cooling rate and thereafter cooling the steel sheet to temperature below 850 C. at various cooling rates as shown by 10 different cooling curves in FIG. 4. From this figure the following facts have been ascertained. When the annealing temperature was 800 C. or 700 C., and the cooling was carried out from the said temperature along the said cooling curves respectively, the higher the annealing temperature, the better the B value on the whole, although there is a peak in the B value, and that the greater the cooling rate in the temperature range below 850 C., the better the B value. When the annealing temperature is higher than 850 C., the cooling from the annealing temperature to 850 C., may be carried out at any cooling rate. If the annealing temperature is higher than l,200 C., the secondary recrystallization will not occur due to the final annealing. On the other hand, if the annealing temperature is as low as 800' C. or 700 C., the B value will not be influenced by the quenching treatment, resulting in a low absolute value lower than 18,500 gausses. Thus, in neither of the latter two cases can the product of the present invention be obtained. From the results of similar experiments made by the present inventors on tests pieces having various Si contents the following conclusions have been obtained about the AlN-precipitating annealing prior to the final cold-rolling.

C should be adjusted to be below 0.080 percent so that a 7- transformation will occur in at least a part of the steel sheet due to the presence of Si in carrying out the annealing prior to the final cold-rolling.

In summary:

when there is to 1 percent Si, there should be less than 0.080 percent C (a steel ingot of less than 0.085 percent C);

where there is 1 to 2.5 percent Si, there should be 0.010 to 0.080 percent C (a steel ingot of0.0l to 0.085 percent C);

where there is 2.5 to 4.0 percent Si, there should be 0.020 to 0.080 percent C (a steel ingot of 0.025 to 0.085 percent C).

l. The C in the steel ingot is 0.005 percent higher than before the annealing prior to the final cold-rolling, because the amount decarburized during the ordinary hot-rolling is taken into consideration. Essentially the C content in the steel sheet is important in carrying out the annealing prior to the final cold-rolling. When reaching the final sheet thickness by a onetime coldrolling method the C content of the steel sheet at the time of carrying out the annealing can easily be caused to be in the specified range as above mentioned, because the said annealing is carried out after the hot-rolling. However, when carrying out a multistage cold-rolling method, as will be described hereinafter, it must be taken into consideration that decarburization may occur, as intermediate annealings are carried out a plurality of times. Also in this case the C content of the steel sheet at the time of carrying out the annealing prior to the final cold-rolling must be regulated so as to be within the specified range as above mentioned.

2. The annealing temperature is in the range of 750 to l,200 C. in which a -ytransformation will occur because of the Si content. In summary the annealing temperature should be:

750 to l,200 C. where there is less than 1 percent Si 850 to l,200 C. where there is l to 2.5 percent Si 960 to l,200 C. where there is 2.5 to 4.0 percent Si The annealing time in this temperature range is 30 seconds to 30 minutes.

When the annealing time exceeds 30 minutes, the growth of crystal grains will occur during the annealing and the development of the secondary recrystal grains in the final annealing will become imperfect. Therefore, it is not favorable. Further, because this annealing is generally carried out continuously, an annealing exceeding 30 minutes is industrially disadvantageous. On the other hand, with an annealing for less than 30 seconds, the effect which is the object of the present invention can not be obtained.

-3. The steel band, the annealing of which has been completed as above mentioned, is then subjected to a cooling which is, however, carried out at any cooling rate within a temperature range, in which 'y formed by the said annealing is transformed into a, that is, a range of 750 to l,200 C., 850 to l,200 C. or 950 to l,200 C. according to the Si content.

4. Thereupon, the steel sheet, in which tit-transformation has been efiected by the said cooling is quenched from the temperature range of 750 to l,200 C., 850 to l,200 C. or 950 to l,200 C. according to the Si content to a temperature below 400 C. by using any adequate artificial means. The cooling time is in a range of 2 seconds to 200 seconds and it is desirable to cool in a shorter time where the Si content is high. in general, the higher the cooling rate, the better the B value irrespective of the Si content. However, according to experiments made by the present inventors, at a cooling rate of shorter than 2 seconds the generation of the secondary recrystallization grains by the final annealing is perfect, but the 8, value is deteriorated. Further, as regards the cooling from the temperature range of 750 to 950 C. down to 400 C. various cooling curves are possible, but in all cases the effect of the present invention can be obtained if the cooling rate at each moment is greater than an average cooling rate, that is,

750 to 400 C./200 seconds (below 1 percent Si) 850 to 400 C./200 seconds (1 to 2.5 percent Si) 950 to 400 C./200 seconds (2.5 to 3.5 percent Si).

Further, as regards the cooling from 400 C. to a lower temperature there is no particular limitation in the present invention. Of course, it is also included in the scope of the present to use the cooling rate of the present invention for the final cooling. However, even when using such a cooling rate below 400 C. there is perceived substantially no influence on the B value of the product. But, in a practical industrial process as a matter of course a steel sheet is always cooled down near room temperature on account of the steel sheet being pickled and cold-rolled after the annealing. Therefore, when carrying out a forced cooling by using any means as above mentioned, the steel sheet is usually cooled from the temperature range of 750 to 950 C. according to the Si content down to near room temperature along a continuous curve. However, in the present invention, because quenching from the quenchingcommencing temperature such as 750 to 950 C. down to 400 C. produces particularly striking effects of promoting the precipitation of desirable MN and in its turn improving the B value of product, the temperature limitations as above mentioned have been specified in the present invention.

5. It is an inevitable condition that AlN should be precipitated at least in an amount of 0.0005 percent (N as AlN) in the steel sheet after the annealing and cooling thereof have been completed.

6. The annealing atmosphere is related to the precipitation of AlN required for the secondary recrystallization as already described. Usually the steel ingot obtained in an open-hearth furnace contains, more than 0.0040 percent N which is sufficient to precipitate 0.0005 percent AlN (N as AlN). Therefore, so long as no great denitrification occurs, the annealing atmosphere can be a reducing or neutral atmosphere such as, for example, H Ar, a gaseou's'mixture of them or air. However, where the ingot is obtained by vacuum melting or the like, the N content will be so small that it will be necessary to add nitrogen during annealing. The method of adding nitrogen is not critical but, in the present invention, it is recommended to carry out the annealing in a neutral or reducing gas containing at least 10 percent N, by volume. According to the opinion of the present inventors, for a precipitate which can selectively grow secondary recrystallization grains having a very well regulated, though not complete, orientation during the final annealing no precipitate is satisfactory other than AlN, as is mentioned at the beginning of the present specification. And the crux of the present invention resides in causing AlN of adequate size to precipitate in the steel sheet before it is subjected to the final heavy cold-rolling. Further, in causing the said AIN to precipitate and controlling the grain size and the distribution thereof the composition of the steel sheet (C, Si and Al) has a close mutual relation with the temperature and time of annealing and the cooling rate through the medium of the y to a transformation.

In the present invention, the cold-rolling is carried out one or more times and the cold-rolling step can be carried out at a reduction rate of 65 to 95 percent depending on the Si content so that, the higher the Si content, the higher the reduction rate. By combining the AlN-precipitating annealing prior to the final strong cold-rolling and the final strong cold-rolling it is possible to obtain a product having a B value of more than 18,500 gausses.

' Any intermediate annealing to be carried out between multistage cold-rolling steps can be carried out at a temperature and for a time which are sufficient to make the coldrolled structure a primary recrystallized structure and these conditions are not set forth in detail. Of course, it is also possible to utilize the above-mentioned AlN-precipitating annealing as an intermediate annealing. In the present invention, the number of times of the cold-rolling step is performed can be determined by the thickness of the hot-rolled sheet and the specified final cold-rolling reduction rate. However, from the industrial technical viewpoint, the hot-rolled sheet is usually 1.5 to 7 mm. thick.

The steel sheet having a sheet thickness after the final coldrolling is then subjected to a decarburizing annealing. This annealing is to make the cold-rolled structure a primary recrystal structure and at the same time to remove C which is detrimental .for developing secondary recrystal grains in the {110} l direction in the final annealing. Any known process can be used for the decarburizing annealing.

The final annealing should be carried out at such temperature and for such a time that secon ary recrystal grains in the {llO} l00 direction can develop well. It is preferable to develop the secondary recrystal grains in a temperature range wherein no y-transformation is produced due to the Si content and at a temperature as high as industrially possible, because the generation of 'y-transformation will change the once-obtained secondary recrystal grains in the {110} l00 direction so that they are in another direction. When Si is less than 1 percent, final annealing should be carried out at 950 C. or usually at a temperature lower than this. However, the higher the Si content, the higher the temperature can be elevated.

When Si is present in an amount more than 2 percent, a temperature higher than l,000 C. is possible. On the other hand, below 800 C., no secondary recrystallization will occur. Unless the temperature is made high, no product having an excellent core loss value can be obtained. Therefore, with a low Si content, the B characteristic is excellent but the core loss is worse than with a high Si content. The annealing time is sufficient if it is more than 1 hour, for the generation of secondary recrystal grains but must be more than 5 hours in order to obtain a product having a low core loss value where there is a high Si content. Further, no matter whether the atmosphere is neutral. reductive or so weakly oxidative that the steel sheet will not be greatly oxidized, a product having a B characteristic of more than 18,500 gausses, which is the object of the present invention, can be obtained. However, in order to obtain a low core loss value when there is a high Si content, it is preferable to carry out the annealing in H However, the specific time and atmosphere has nothing to do with the substance of the present invention.

EXAMPLE 1 An al-killed steel ingot containing 0.050 percent C and 0.041 percent Al was bloomed and hot-rolled to a hot-rolled steel sheet 2.2 mm. thick. After the steel sheet was annealed in N at 1,000 C. for 2 minutes, it was cooled in warm water having a temperature of C. The cooling rate was about 10 seconds for the temperature descent from l,000 C. to 750 C. and about 25 seconds from 750to 100 C. The content of AlN after the annealing was 0.0045 percent (N as AIN). After pickling, the sheet was cold-rolled at a reduction rate of 77.3 percent to make the thickness of the sheet 0.50 mm. The coldrolled sheet was then decarburized at 750 C. for 5 hours by an open coil system and thereafter finally annealed in H at 870 C. for 20 hours.

The excitation characteristics of the product were as follows:

u =l9,950 gausses w,,,,,, =3.60 W/kg.

EXAMPLE 2 A silicon steel ingot containing 0.32 percent C, 1.05 percent Si and 0.036 percent Al was hot-rolled to a hot-rolled steel sheet 2.2 mm. thick. The content of C in the hot-rolled steel sheet was 0.030 percent. After the steel sheet was annealed in N: at l,050 C. for 2 minutes, it was cooled by slightly spraying N gas onto the both surfaces of the steel sheet. The cooling rate was about 13 seconds for the temperature descent from l,050 to 850 C. and about 70 seconds for that from 850 to 400 C. The content of AlN after'this annealing was 0.0062 percent (N as AlN). After pickling, the steel sheet was then cold-rolled at a reduction rate of 84.1 percent to a cold-rolled steel sheet 0.35 mm. thick. After the cold-rolled sheet was decarburized in wet H, at 800 C. for 3 minutes, it was finally annealed in H at 950 C. for 10 hours.

The magnetic characteristics in the rolling direction of the product were as shown by FIG. 18, that is,

m =19,700 gausses m 1.85 W/kg.

EXAMPLE 3 A silicon steel sheet containing 0.043 percent C, 2.10 percent Si and 0.036 percent Al was bloomed and hot-rolled to a hot-rolled steel sheet 3 mm. thick. The content of C of the hotrolled sheet was 0.041 percent, indicating that only a slight decarburization was effected. At first, the hot-rolled steel sheet was cold-rolled at a reduction rate of 30 percent make the thickness of the sheet to 2.1 mm. Then, it was annealed in N, at 1,100" C. for 2 minutes, and thereafter cooled by blowing a jet air steam against the sheet. The cooling rate was about 18 seconds for the temperature descent from l,l00 C. to 850 C. and about 27 seconds for that from 850 to 400 C. The content of AlN after this annealing was 0.0055 percent (N as AlN). Thereupon, the steel sheet was cold-rolled at a reduction rate of 83.3 percent to a cold-rolled steel sheet 0.35 mm. thick. After the cold-rolled steel sheet was decarburized in wet H, at 800 C. for 3 minutes, it was annealed at 1,200 C. for 20 hours. The magnetic characteristics in the rolling direction of the product were as shown by FIG. 1C, that is:

8 -19,570 gaunes u, -i.is W/kg.

EXAMPLE 4 A silicon steel ingot containing 0.045 percent C, 2.05 percent Si and 0.020 percent Al was bloomed and hot-rolled to a hot-rolled steel sheet 2.3 mm. thick. The content of C of the hot-rolled steel sheet was 0.041 percent. After the hot-rolled steel sheet was annealed in N at 1,050 C. for 2 minutes, it was cooled by slightly blowing N gas onto the steel sheet. The cooling rate was substantially the same as in example 2, and the content of AlN after this annealing was 0.0032 percent (N as AlN). Then, the steel sheet was cold-rolled at a reduction rate of 84.8 percent to a cold-rolled sheet 0.35 mm. thick. Thereafter, the cold-rolled steel sheet was decarburized in wet H at 800 C. for 3 minutes and then finally annealed at 1,200 C. for 20 hours. The magnetic characteristics in the rolling direction of the product were:

=l 8,800 gausses =1.2$ W/kg.

io is/so EXAMPLE 5 A silicon steel ingot containing 0.043 percent C, 2.96 percent Si, 0.029 percent Al, 0.10 percent Mn and 0.029 percent S was bloomed and hot-rolled to a hot-rolled steel sheet 2.8 mm. thick. The contentof C after the hotrolling was 0.040 percent. After the hot-rolled steel band was continuously annealed in N, at 1,l50 C. for 2 minutes, it was subjected to a slow cooling to 950 C. in the cooling zone of a furnace, which was followed by a quenching by spraying with high-pressure water. The cooling rate was about 20 seconds for the temperature descent from l,l50 to 950 C. and about 9 seconds for that from 950 to 20 C. The content of AlN after this annealing was 0.0040 percent (N as AlN). After pickling, the steel sheet was then cold-rolled at a reduction rate of 87.5 percent to bring the thickness of the sheet to the final gauge of 0.35 mm. After the cold-rolled steel sheet was continuously decarburized in wet H at 850 C. for 3 minutes, it was finally annealed in H at 1,200 C. for hours. The excitation characteristics in the rolling direction of the product were as shown by FIG. ID, that is:

indicating an excellent orientation and core loss value. FIG. 2 shows the core loss characteristics of this example as compared with those of a known commercial product, which is shown by FIG. 2B.

EXAMPLE 6 A silicon steel ingot containing 0.050 percent C, 3.15 percent Si, 0.035 percent S and 0.021 percent Al, said ingot being prepared in an open-hearth furnace, was bloomed and hotrolled to a hot-rolled steel sheet 3.0 mm. thick. After the hotrolled steel sheet was maintained in a continuous annealing furnace having N atmosphere at 1,050 C. for 1 minute, it was subjected to a forced cooling by using an N -gas-blowing device installed at the port of the furnace. After pickling, the steel was cold-rolled at a reduction rate of 51 percent to a cold-rolled sheet having an intermediate gauge as of 1.47 mm. Thereupon, the cold-rolled steel sheet was annealed at 800 C. for 1 minute to effect the primary recrystallization and then subjected to the final strong cold-rolling at a reduction rate of 84.5 percent to a cold-rolled sheet having the final gauge of 0.228 mm. which was then continuously decarburized and finally annealed at l,200 C. for 20 hours. The magnetic characteristics in the rolling direction were as follows:

BID =l 9,300 gausses is/so =0.72 W/ltg.

We claim:

1. In a process for producing a single-oriented electromagnetic steel sheet having an excellent excitation characteristic and a high saturated magnetic induction in the rolling direction of the steel sheet, which process includes the steps of: subjecting a steel ingot containing 0 to 4.0 weight percent Si, less than 0.085 weight percent C and 0.010 to 0.065 weight percent acid-soluble Al to blooming and hot-rolling; subjecting the hot-rolled steel sheet to: (a) at least one annealing, said one annealing when it is the only annealing being an annealing for causing at least 0.0005 weight percent AlN (N as AlN) to precipitate before a final cold-rolling, and when there is a plurality of annealing treatments, one of the treatments being an annealing for causing at least 0.0005 weight percent AlN (N as AIN) to precipitate before a final cold-rolling; and to (b) at least one cold-rolling, said one cold-rolling when it is the only cold-rolling and one of the cold-rollings when there is a plurality of cold-rollings being a final cold-rolling subsequent to said one annealing and being at a reduction rate of 65 to percent according to the Si content to reduce the thickness of the steel sheet to the final gauge; and then subjecting the steel sheet to a decarburizing annealing and a final finishing annealing at a temperature of at least 800 C.; the improvement comprising carrying out said one annealing before the final coldrolling for precipitating MN for causing the 'y-transformation in a part of the steel sheet by heating the steel sheet to an annealing temperature of from 750l ,200 C. when the Si content is up to 1 weight percent and the C content is up to 0.080 weight percent, to a temperature of 850-l ,200 C. when the Si content is from 1.0 to 2.5 weight percent and the C content is from 0.010 to 0.080 weight percent, and to a temperature of 950-l,200 C. when the Si content is from 2.5 to 4.0 weight percent and the C content is from 0.020 to 0.080 weight percent, holding the sheet at the annealing temperature for a time of from 30 sec. to 30 min., and then quenching the annealed sheet to a temperature at least as low as 400 C. in a time of from 2 to 200 sec.

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